TGA
is one of the simplest instruments available for thermal characterization, and
the analysis results offer a wealth of information about the tested material.

Many different thermal techniques can be employed for
material characterization, including thermomechanical analysis (TMA), dynamic
mechanical analysis (DMA), differential scanning calorimetry (DSC), and
thermogravimetric analysis (TGA). These complementary techniques are used for
compositional and structure elucidation, and for investigating the viscoelastic
properties of materials.

During the production and processing of ceramic and glass materials, various
parameters (including chemical composition and microstructure of various
phases, as well as their distribution) are tightly controlled to impart the
desired mechanical and chemical properties to the end product. The knowledge of
thermal stability and reaction kinetics of various phases present in the
ceramic and glass materials is of prime importance.

Thermal characterization and testing are
employed to evaluate and optimize the chemical and physical properties of
ceramic and glass materials. For example, thermal analysis could be used to
investigate the pyrochemical reactions of clay-based ceramic materials.
Pyrochemical reactions include events such as loss of bound (water of crystallization)
and unbound (free) water, loss of organic matter, crystal structure
transformation (polymorphism), and decomposition. These thermal events can
easily be characterized using TGA and DSC analytical techniques.

Figure
1. TGA thermogram of calcium carbonate.

Technique Characteristics

Some thermal techniques are used in conjunction with
fundamentally different, non-thermal spectroscopic techniques for a better
understanding of the chemical properties. For example, TGA is combined with
mass spectrometry (TGA-MS) and Fourier transform infrared spectroscopy
(TGA-FTIR) to investigate the decomposition of products, especially evolved
gases of polymer materials.

When using TMA, the test specimen is subjected to a minimal load, and changes
in length or volume are measured at varying temperatures. The deformation load
could be in compression, tension or bending mode. TMA is commonly used to
characterize the thermal expansion and shrinkage that takes place during the
processing (sintering) and use of ceramic and glass materials. Proper characterization
of thermal expansion and shrinkage behavior of these materials is of paramount
importance, especially when such materials are used in combination with
different materials, such as polymers and metals. Any significant mismatch in
the thermal expansion behavior of the two materials could potentially lead to a
product failure.

With DMA, the mechanical properties of the sample are measured as a function of
time, temperature and frequency. The strain
is recorded as the test specimen is subjected to an oscillatory stress, which
can be in compression, tension, shear or flexure mode. This technique is
routinely used in the investigation of the viscoelastic behavior of polymer
materials. DMA is especially useful for characterizing the a, b and g transitions
of the polymers, which are difficult to detect with other techniques.

In DSC analysis, the heat flow into a sample
and a reference material is measured as a function of temperature and time.
This technique is routinely employed to characterize various thermal events in
polymers, such as glass transition, melting, crystallization, heat capacity,
phase transformation, oxidative stability and curing reaction. DSC is also
commonly used to measure the specific heat capacity of ceramic and glass
materials. The reaction kinetics and phase transformations that take place
during the processing and use of these materials can also be characterized
using this technique.

TGA is one of the simplest
instruments available for thermal characterization, and the analysis results
offer a wealth of information about the tested material. The technique is used
primarily to investigate the thermal and oxidative stability, and the inorganic
and organic composition, of the material.

During TGA analysis, the weight of the sample is measured as a function of time
and temperature in an inert or oxidative atmosphere. When a sample is heated in
an inert nitrogen atmosphere, the thermal decomposition of the components of
the sample occurs at different temperatures based on the sample’s thermal
stability. Most of the organic materials are thermally decomposed completely
below 600°C; the residual noncombustible mass remaining at the end of heating
to 800-900°C in an oxidizing atmosphere consists of inorganic materials. TGA is
used to follow the reaction kinetics and phase transformations that take place
during the processing or use of ceramic and glass materials.

A typical TGA thermogram is shown in Figure 1, which shows the decomposition
profile of a reference material, calcium oxalate monohydrate
(CaC2O4 ·
H2O). Three distinct weight-loss steps are evident and
correspond to the loss of water of crystallization (25-230°C), decomposition of
dehydrated calcium oxalate into calcium carbonate and carbon monoxide
(230-600°C), and, finally, the decomposition of calcium carbonate into calcium
oxide and carbon dioxide (600-800°C).

Calcium Carbonate Analysis

Calcium carbonate (whiting) is used in varying amounts as a
flux material in the production of ceramic materials, and ceramic tile
adhesives also contain a significant amount of calcium carbonate. The level of
calcium carbonate in a product should be optimized to get the desired results
and ensure that the primary function of the product is not compromised.

TGA is a beneficial quality control technique used to determine the amount of
calcium carbonate (CaCO3) present in a product. When
heated in an inert nitrogen atmosphere, calcium carbonate decomposes into
calcium oxide (CaO) and carbon dioxide (CO2) at
approximately 700°C:

CaCO3 → CaO
+ CO2

Theoretically, the decomposition of calcium carbonate results in a mass-loss of
approximately 44% by weight. Since the organic/polymeric material decomposes completely
at temperatures well below 600°C, any mass-loss that occurs in the temperature
range of approximately 600-800°C can be attributed to the decomposition of the
calcium carbonate material in the sample.

Once the mass-loss associated with the decomposition of calcium carbonate is
determined, the amount of calcium carbonate present in the sample is
calculated. However, care must be taken to make sure that the sample does not
contain any other inorganic materials that decompose in the same temperature
range.

Figure
2. TGA thermogram of simethicone.

Simethicone Analysis

TGA can be used as a quality control technique to determine
the weight percent of silica in simethicones. By definition, simethicones are
poly(dimethyl siloxane) (PDMS) polymers that contain 4 to 7% by weight silica.
Silica is one of the basic components of clay-based ceramics.

In a typical TGA analysis of a simethicone, approximately 20 mg or more of the
sample is heated from room temperature to 800°C in an inert nitrogen atmosphere
(flow rate approximately 200 mL/min). The amount of residue (silica) remaining
at the end of heating is determined from the TGA thermogram of the sample.

Figure 2 shows the decomposition profile of a
simethicone product. As is evident, the catastrophic decomposition of the PDMS
polymer begins at approximately 400°C, and the sample contains approximately
5.2% by weight inorganic material (silicon dioxide).

Carbon Black Analysis

Carbon black is used extensively in significant amounts in
rubber and ink formulations, and elemental carbon is also used in the
manufacture of carbon/silicon carbide/glass composite materials. The presence of an optimum level of carbon
black in these formulations is critical for the desired performance of a
product.

TGA analysis is routinely used as a quality control technique to determine the
amount of carbon black present in polymer and ink materials. In this technique,
the material containing carbon black is heated to approximately 700°C in an
inert nitrogen atmosphere to burn off the organic/polymer materials. Then an
oxidizing air atmosphere is introduced to oxidize the carbon black.

The mass-loss associated with the oxidation of carbon black is determined from
the TGA thermogram, and the amount of carbon black present in the sample is
calculated. Figure 3 shows the decomposition profile of an ink product and
indicates that it contains approximately 4.1% by weight of carbon black.

Effective Characterization

Various
thermal techniques can be employed to characterize ceramic and glass materials.
Each of these thermal techniques provides unique information that can be used
to optimize the thermal and mechanical properties of the end product.